The field of the present invention relates to semiconductor-based photodetectors. In particular, a reflector for a double-pass photodetector is disclosed herein.
Semiconductor-based photodetectors are well known, and typically comprise multiple layers of suitably doped semiconductor materials formed on a substrate. The doped photodetector layers are arranged so that absorption of light results in generation of charge carriers, which in turn gives rise to electrical signals substantially proportional to the amount of light absorbed. The efficiency or responsivity of the photodetector is proportional to the absorbance of its layers. In common detector geometries, it is often the case that the light to be detected propagates outside the substrate before impinging on the photodetector layers. In this front-illuminated arrangement, incident light that is not absorbed by the photodetector layers propagates into the substrate and is lost.
In other detector geometries, the light to be detected propagates within the substrate before impinging on the photodetector layers. Examples of such detector geometries may be found in, for example, in: i) Fukano et al, Electron. Lett. Vol. 32 No. 25 p. 2346 (December 1996); ii) Fukano et al, J. Lightwave Tech. Vol. 15 No. 5 p. 894 (May 1997); iii) Fukano et al, J. Lightwave Tech. Vol. 18 No. 1 p. 79 (January 2000); iv) Kato et al, IEEE Photonics Tech. Lett. Vol. 11 No. 6 p. 709 (June 1999); v) U.S. Pat. No. 6,353,250; vi) U.S. Pat. No. 6,770,945; and vii) U.S. Patent Pub. No. US-2004-0129935-A1. In this back-illuminated arrangement, incident light that is not absorbed by the photodetector layers either escapes the photodetector and substrate, or is incident on any layers that may be present over the photodetector layers.
In cases wherein detector responsivity is at a premium, or the absorbance of the photodetector layers is lower than may be desired at a wavelength of interest, a back-illuminated photodetector geometry affords an opportunity to increase overall photodetector responsivity. Therefore, it may be desirable to form a reflector over the photodetector layers for reflecting at least a portion of light transmitted by the photodetector layers for a second pass through the photodetector layers.